The present invention relates to laminate structures, methods for making such structures, and various uses therefor. In a particular aspect, the present invention relates to foam core assemblies useful for the preparation of microwave circuit assemblies, particularly antennas.
A variety of composite structures are used in the electronics industry. Important considerations in the formation of such structures include the structural integrity of the finished structure, the ability of the resulting structure to withstand a variety of processing conditions after assembly thereof (e.g., processing conditions typical of printed circuit manufacture), the performance properties of the finished structure (e.g., the dielectric constant, resistance to moisture and other potentially harsh exposures, and the like), competitive cost, ease of manufacture, and so on.
Historically, in the production of printed circuit boards used as antennas and other elements of cellular and wireless infrastructure, conventional woven glass laminates based on PTFE (polytetrafluoroethylene) have been utilized. While these are readily and conveniently manufactured using technology well known to those of skill in the art, they tend to be expensive, are heavy (having a density of about 2.5) and are limited to a dielectric constant no lower than about 2.17.
There is, however, a growing need for low cost, lightweight antennas with dielectric constants lower than about 2.0, preferably closer to 1.5. In attempts to achieve this, the prior art teaches several variations on the use of rigid foams combined with metal foils.
For example, Gosselin (U.S. Pat. No. 5,733,639 describes methods for making a structure in which a microballoon-filled adhesive is used to bond metal foil directly to a rigid polyisocyanurate foam. While potentially useful in manufacturing individual antennas, the method is limited in that there is no true barrier to attack of the foam surface by process chemistries (both aqueous and organic) typical of printed wiring board manufacturing processes once the copper has been etched away. This results in degradation of and/or inconsistency in electrical properties and performance. In addition, the Gosselin method is not amenable to the high volume continuous manufacture necessary to produce product economically. Maoz et. al. (U.S. Pat. No. 5,541,366) teaches a similar approach without, however, specifying a particular adhesive to bond the copper to the foam core.
When it was attempted to resolve the issue of degraded electrical performance by using a polyurethane film adhesive to bond copper foil directly to a rigid Baltek polystyrene foam core at 350xc2x0 F., doing so led to the partial structural collapse of the foam and did not result in an impermeable barrier between the copper and the foam. The resultant product had pinholes in the film/bonding layer, which resulted in the penetration of etch chemicals during processing. This confirmed the concern noted above with respect to the limited applicability of the Gosselin et. al. methodology.
In a similar experiment the foam itself was coated with a ceramic-filled resin system known to have good electrical properties. That system was B-staged for 3 minutes at 139xc2x0 C. and then laminated for 90 minutes at 360xc2x0 F. and 300 psi (typical PWB laminate manufacturing techniques). Again the foam collapsed due to heat and pressure, resulting in a material that was too dense and the seal between the copper and the foam was still inadequate to eliminate etchant penetration and entrapment in the foam structure. Clearly another approach to the problem is still needed.
Among the alternative approaches described in the art, Metzen et. al. (U.S. Pat. No. 6,356,245) teaches the use of two rigid foam layers bonded on opposite sides of a pre-formed microwave circuit (specifically a transmission line used as part of a beam-forming network for a microwave antenna), wherein the pre-formed circuit is formed on a piece of traditional laminate material as noted above. Such an approach is of limited applicability as it requires substantial assembly work after the circuit has been formed. Thus, while the Metzen methodology may be useful for forming a buried stripline circuit, it could not be used, for example, to form a surface microstrip pattern, such as is commonly found in antennas for cellular transmitter/receiver antennas.
Other functional, but technically problematic approaches have also been described in the art, such as for example, by first manufacturing a flexible printed circuit on a commercially available product such as Dupont""s Pyralux(copyright) and then bonding that circuit to a foam core using adhesive. A critical deficiency in such an approach is especially notable when circuitry needs to be formed on both sides of a finished assembly, since critical and precise alignment of circuit features from side to side is difficult at best when gluing individual etched circuits on opposed sides of a piece of foam. Issues such as foam collapse (as described above) are also expected to arise when employing such an approach.
In summary, the prior art clearly does not provide a ready-to-use assembly that can be supplied to a printed circuit manufacturer and immediately subjected to conventional processing. For example, the prior art does not provide a barrier impervious to process chemicals and is, moreover, largely limited to the use of rigid foam cores to which there must be a secondary application of copper foil (which can easily result in a permeable surface) or pre-etched circuitry, which adds cost and complexity.
Accordingly, there is a clear need in the art for low cost, readily manufacturable, lightweight, low dielectric constant composite structures (and methods of making same) which can be supplied with conductive metal foils on one or both sides, have provision for an impervious barrier to protect the underlying core material from attack by process chemistries, and can be subjected to a variety of industrial processing conditions, and thereby used in a variety of electronic applications. The present invention satisfies these and related needs, as described in greater detail in the following specification and claims.
In accordance with the present invention, there are provided novel assemblies which are useful for a variety of microwave/RF applications. Invention assemblies have low dielectric constant, making them suitable for use in a variety of electronic applications. In addition, invention assemblies are resistant to attack by acidic aqueous media, basic aqueous media and/or organic media, making it possible to subject such assemblies to a variety of processing conditions commonly used in PCB manufacturing, such as, for example, chemical etching to introduce circuitry thereto.